where \(c\)  is a constant. Using the language developed in Refs. [1]}, [2]}, [3]}, we can say that \(\mathcal {L}\)  is an homogeneous pseudo-differential operator of degree 0. From a physical point of view, the picture gets very clear in Fourier space, while writing Eq. REF  in a equivalent way as
\(\mathcal {F}[\mathcal {L}u](t,k) =-c\partial _k\widehat{u}(t,k),\) 
